Mon Mothma mentions Bothan spies in Return of the Jedi and with no further mention of their kind in the rest of the films, one could easily remain confused forever. That’s where the expanded universe comes in.
A species from Bothawui, Bothans are renowned as the best spies in the galaxy. Prior to the battle of Endor, Bothan spies captured an Imperial freighter carrying highly classified data about the second Death Star. New Republic Councilor Borsk Fey’lya is a Bothan who was born on the Bothan colony world of Kothlis.
The Bothan spynet is second to none, with operatives throughout the galaxy. Even though the spynet was used primarily to support the Rebel Alliance, the Empire and the criminal underworld occasionally found the spynet useful for their own purposes. The spynet’s work often put its members at great risk.